Disc jet electropolishing apparatus and method

ABSTRACT

Jet electropolishing apparatus and method for preparation of thin foil specimen discs for transmission electron microscopy, including means responsive to passage of electrolyte through the specimen for terminating both the flow of electric current and the flow of electrolyte.

United States Patent Larson et al.

[151 3,657,083 1 Apr. 18,1972

Appl. No.: 76,678

U.S. Cl ..204/l40.5, 204/228, 204/229,

204/273 lnt. Cl. ..C23b 3/06, BOlk 3/00 Field of Search ..204/l40.5, 228, 229, 273

[56] References Cited UNITED STATES PATENTS 2,767,137 10/1956 Evers ..204/l43 R 3,075,902 l/1963 Bradley et a]... ..204/229 3,293,162 12/1966 Sullivan ..204/140.5 3,434,956 3/1969 Glenn ..204/ 140.5

Primary Exa miner-John H. Mack Assistant ExaminerT. Tufariello Attorney-Brumbaugh, Graves, Donohue & Raymond 5 7] ABSTRACT Jet electropolishing apparatus and method for preparation of thin foil specimen discs for transmission electron microscopy, including means responsive to passage of electrolyte through the specimen for terminating both the flow of electric current and the flow of electrolyte.

7 Claims, 3 Drawing Figures PATENTEDAPR 18 m2 SHEET 1 BF 2 INVENTORS JAY MICHAEL LARSO DOUG HUGH POLO 8x RAYMO TAGGART BY SIn-JO L G M,

Their ATTORNEYS PAIENIEIIIPII I8 III: 3, 657, 083

I SHEET 2 BF 2 I20 VOLTS, 6O CYCLE AC. POWER F/G. Z

sNAP' ACTION I RELAY POWER 26 VOLT 0.0. AMPLIFIER POWER SUPPLY AND BELL ELECTROLYTE Q? REsERvoIR PLJMP JET 2; j NOZZLE I I ASS'BLY 30 VARIABLE 0.0. POWER SUPPLY F X 43 .45HMILLIAMMETER g II 1N VENTURS -111. JAY MICHAEL LARSON,

oou As HU H P0 oms a 44 POWER RAYii oND T seAI iT SUPPLY BY ,W (AM,

(3W *WM Their ATTORNEYS DISC JET ELECTROPOLISHING APPARATUS AND METHOD This invention relates to apparatus for jet polishing thin foil specimen discs to be used in transmission electron microscopy studies and to the method by which the specimen discs are prepared.

Jet polishing techniques for the preparation of thin foil specimens for examination by transmission electron microscopy have been in use for several years and possess a number of advantages over the more conventional methods of electropolishing. However, during the final stages of jet polishing, the perforation of a foil occurs so rapidly that by the time the flow of current and of the jet stream are shut off, much of the foil area suitable for examination in the electron microscope has been dissolved.

One approach taken by the prior art to overcome this problem is by focusing a light source on one side of a specimen disc and by observing the other side through a microscope. When light is observed through the foil, an operator manually shuts off the power. As can be imagined, this procedure is time-consuming and tedious for an operator, and results are not easily duplicated. A photoelectric cell can be used in place of the microscope as a device to detect the perforations in the foil that signal the final stages of the polishing process. This procedure, however, requires extremely careful focusing and alignment of equipment as well as a considerable amount of sophisticated electronic equipment. Other automatic thinning techniques and apparatus are available, but these require optical as well as electrical devices, and have a number of disadvantages.

The present invention provides apparatus of improved sensitivity for controlling the termination of the thinning process. By following the method of the invention, it is possible to produce perforations in disc specimens ranging from I to microns in diameter in a consistent and automatic manner, while using simple electronic equipment.

The nature of the invention and its objects, novel features and advantages will be seen from the following detailed description of the embodiments illustrated in the accompanying drawings in which:

FIG. 1 illustrates the general arrangement of the jet nozzle assembly of the present invention;

FIG. 2 is a schematic drawing of the apparatus of the present invention; and

FIG. 3 is a diagram of the circuit used in the relay power amplifier shown in FIG. 2.

Referring now to the drawings, FIG. 1 shows the general arrangement of the apparatus for electropolishing disc specimens. The apparatus comprises a stainless steel body portion 10, having a seat 11 therein adapted to receive a disc specimen 12. The stainless steel body portion 10 may be supported with a clamp on a ringstand suitably insulated (not shown) or by any other conventional means. The body portion 10 contains a centrally disposed channel 13, and the channel 13 is preferably lined with a Teflon insert 14.

Mounted above the body portion 10 is an insulating connector 15, suitably of Teflon, having a central passage 16 therein, positioned immediately above the channel 13 in the body portion 10. Mounted above the insulating connector is a stainless steel top portion 17 having a centrally disposed opening 18 adapted to receive a stainless steel set screw sensing device 19 with lock nut 20.

Mounted below the body portion 10 is an insulating base member 21, suitably of Teflon, having a centrally disposed channel 22 approximately coextensive with and aligned with the surface of the disc specimen 12. An O-ring 23, of a suitably resilient and electrolyte-resistant material is provided in recess 24 on the upper surface of base member 21. The O- ring 23 is adapted to form a seal around the periphery of the disc specimen 12, the disc 12 lying between O-ring 23 and a seat 11 in body portion 10.

Depending from the base 21 is a tubular glass tee 25, having mounted therein a glass nozzle 26 with a winding of platinum wire 27. The glass nozzle 26 is connected to a supply of electrolyte (not shown) by tygon tubing 28. The nozzle 26 is retained in position within the glass tee 25 by means of at least two O-rings 29. These O-rings also provide means for adjusting the distance between the disc 12 and the tip of nozzle 26, and provide a seal to prevent passage of electrolyte. The tubing 29a collects electrolyte and returns it to the electrolyte reservoir; see FIG. 2.

The negative lead from a power supply (shown in FIG. 2) is connected to the platinum wire 27 at point 30. The positive lead from the power supply is connected to the stainless steel body portion 10 at point 31. Point 32 of stainless steel top portion 17 is connected to the positive lead of the relay power amplifier shown in FIG. 2. It will be noted that the connector 15 effectively insulates the stainless steel top portion 17 from the body portion 10 and the disc specimen 12.

The polishing apparatus and the auxiliary equipment are shown schematically in FIG. 2. The pump 40, preferably a Ministaltic pump suitable for pumping acids and having its internal parts coated with Tygon, circulates electrolyte through the system. The reservoir 41 maintains the electrolyte at a constant temperature. Under some circumstances, no cooling apparatus is required, but where necessary, a suitable cooling apparatus such as the U-tube cooling apparatus, suggested by Davies, J. Inst. Metals, 96 (1), pp. 61-62. 1968, may be incorporated.

When actuated, the pump 40 carries electrolyte from the reservoir 41 to glass nozzle 26, shown in more detail in FIG. 1. The electrolyte is directed from the tip of the nozzle 26 against the surface of the disc specimen 12 in laminar flow. As recognized by those skilled in the art, turbulent flow of the electrolyte must be avoided to prevent undesired preferential etching of the disc at its periphery. The electrolyte is returned through tube 29 to the reservoir 41.

The snap action relay power amplifier 42 is used to terminate the flow of electric current from the disc specimen 12 to the cathode 27 and the flow of electrolyte from the glass nozzle 26 at the instant that the disc 12 is perforated. Immediately on perforation, the electrolyte passing through the hole in the disc 12 makes contact with the tip of the sensing device 19 to actuate the snap action relay power amplifier 42. The sensing device 19 and relay power amplifier 42 cooperate to form a means responsive to passage of electrolyte through the specimen for terminating both the flow of electric current and the flow of electrolyte. At the same time, a bell is provided to signal the finish of the thinning process to the operator.

The current for the electropolishing portion of the process is supplied from a variable D.C. power supply 43. A constant D.C. power source 44 supplies power to the snap action relay power amplifier 42. A milliammeter, 45, is supplied in the line connecting the variable D.C. power supply 43 to terminal 31 on the body portion 10.

FIG. 3 gives a circuit diagram of a typical snap action relay power amplifier of the type shown at 42 in FIG. 2. The device comprises the following components:

a PNP Transistor 2N2906, 51,

a NPN Transistor 2N222 l 52,

a Resistor (40 OHM), 53,

a Resistor (500 OHM), 54,

a Resistor (5K OHM), 55,

a Diode IN645, 56,

a Zener Diode (3.9 volt), 57,

a Zener Diode (20 volt), 58 and a Relay 12 volt (200 OHM), 59.

This device terminates both the current flowing from the disc to the cathode and the jet stream at the instant the disc is perforated.

EXAMPLE I A nickel alloy was used to illustrate the present invention. The alloy was cold rolled to form a 0.5 mm thick sheet and 3.1 mm discs suitable for the specimen holder of a JEM 7 electron microscope were cut using a No. 5 Whitney punch. Examination of these discs without the use of grids in a Phillips 300 electron microscope is possible when using a special holder designed by Strutt, J. Sci. Instrum. 32, p. 411. 1961.

The faces of the discs 12 were mechanically polished prior to mounting in the polishing apparatus to insure a good seal between the disc 12 and the O-ring 23.

The electrolyte used for thinning nickel alloys consisted of 39 parts of concentrated H 80, and 29 parts of distilled H O, as described in Metal Progress, July 15, 1954, p. 171. The electrolyte was circulated at a rate of 0.3 ml/sec at 20 C. through a 0.5 mm diameter jet. A jet orifice one-third the diameter of the polished area gave the optimum polishing conditions for Ni-Ta alloys. The specimen 12 was maintained at 10 volts relative to the 0.005 inch diam. platinum wire cathode 27, resulting in a current density of 350 mA/cm at the specimen surface. The current was regulated by adjusting the distance between the tip of glass nozzle 26 and the specimen 12, either by moving the base 21 or the glass nozzle 26 with respect to the glass tee 25. The milliammeter is used to monitor the current and the adjustment was made to obtain a maximum amount of current. The positive lead from the power source is connected to point 31 on body portion 10 and the negative lead is connected to point 30 as shown in FIG. 1.

The first side of the disc 12 is electropolished for 8 minutes under the conditions given above, after which the base 21 is removed from the glass tee, and the electropolished side of the disc is rinsed in place with dry methanol. The disc 12 is then removed from the holder and dried with lens paper. The disc 12 is then reversed to expose the unpolished side to the jet.

The extent to which the first side of the foil is electropolished will depend upon the materials used and the rate of flow of electrolyte, and is generally determined empirically for each set of conditions and materials. It is generally desired to polish to the extent that about half of the thickness of the foil in the center is removed.

Before electropolishing the second side of the disc 12, the connector 15, stainless steel top 17, and stainless steel screw 19 are placed in the position shown in FIG. 1. An ohmmeter is connected across points 30 and 32, and the screw 19 is positioned so that its tip makes contact with the specimen disc 12. The screw is then backed off slightly so that a gap, effectively an infinite resistance, exists between the tip of screw 19 and the specimen 12, after which the screw 19 is secured in place by the lock nut 20.

The snap action relay power amplifier is designed to activate the relay and thus cut off the power when it monitors a potential greater than 4 volts across the points 31 and 32. The disc 12 is polished until perforation occurs, at which time the acid penetrates the foil and acts as a conductor between the specimen 12 and the tip of screw 19. The voltage drop across the input of the snap action relay power amplifier, points 31 and 32, goes immediately from volts to the electropolishing potential (points 31 and 30) and activates the relays, turning off the current and pump and activating the bell.

At the completion of the thinning step, the base 21 is removed from the glass tee 25, the top 17 is unscrewed, and the disc 12 is rinsed in place with dry methanol after which it is removed, washed and dried. The polished disc 12 can be stored in a proper environment or placed directly in the electron microscope holder for examination.

The fine circular perforations that have been produced in Ni alloys in accordance with the invention are l-p. in diameter. For 0.5 mm thick discs, an area of approximately 1,000p. can be produced for observation at 100 Kv in the electron microscope. The area available for electron transmission can be increased by starting with a thinner disc.

After the first penetration of a foil, further thinning can be conducted, if the point of initial perforation is covered by a lacquer such as Microstop. In this way, repeated polishing can be carried out to produce other areas for examination with the electron microscope. The first perforation occurs in a 0.5 mm thick disc in approximately minutes and further perforations can be produced in about 2 minutes.

If it is desired to increase the rate of thinning, the voltage and the flow rate of the electrolyte can be increased, particularly in the initial stages of thinning. If turbulent flow in the jet stream is not avoided the edges of the specimen may be etched preferentially by the electrolyte resulting in unsuitable perforations around the edge of the foil. The size of the perforation can be varied in Ni-base alloys by adding methanol or glycerin to the electrolyte, thereby changing its viscosity and flow properties. The size may also be varied by adjustment of the gap between the specimen surface and the tip of screw 19. A wider gap permits a larger amount of electrolyte to penetrate before the termination of current and electrolyte flow, resulting in larger perforations.

This automatic apparatus considerably enhances the capabilities of the disc jet polishing technique. Using this extremely rapid method, it is possible to control the final stages of thinning so that usable thin foils having a substantial area for electron transmission studies can be produced consistently.

The apparatus and method of the present invention has been used effectively to thin foils for electron microscopy examination in the following alloy systems:

Ti, with 5 to 15%Cr;

Ni, with 0 to 34%Ta;

Ni, with O to 20%Nb;

Ni, with 10 to 20% Va;

Cu, with 11%Ge, and

Maraging Steels.

In each instance an electrolyte known by those skilled in the art as effective for a particular foil composition was used. Other materials, including MgO, GeAs and other non-conducting or semi-conducting materials may be thinned in accordance with the present invention. For materials that use other than electrolyte, such as hot acid, the apparatus of the present invention can be used to monitor the voltage between the screw sensing device and the cathode in the hot acid. In this event, the thinning process is stopped when hot acid perforates the material.

The method of the present invention has applications other than making thin foils for electron microscopy. The process can be used to make a circular hole of a given diameter consistently in the range of between 1.0 and microns. A typical use would be the manufacture of apertures used in electron microscopes.

We claim:

I. In an apparatus for thinning a foil specimen including means for securing the specimen, means for directing a thin stream of polishing electrolyte against one surface thereof in laminar flow while recovering and recirculating the electrolyte, and means for establishing a flow of electric current through the stream of electrolyte, between the specimen as an anode and a cathode, the improvement comprising the provision of means responsive to passage of electrolyte through the specimen for terminating both the flow of current and the flow of electrolyte.

2. The apparatus of claim 1, wherein the means responsive to passage of electrolyte through the specimen for terminating both flow of current and the flow of electrolyte includes means for determining a voltage drop caused by the electrolyte establishing contact between the specimen and sensing means.

3. The apparatus of claim 1 wherein the means responsive to passage of electrolyte through the specimen for terminating both flow of current and the flow of electrolyte comprises a conductive sensing device adapted to be positioned immediately adjacent one surface of the specimen and a snap action relay power amplifier connected with the sensing device.

4. A process for thinning a foil specimen comprising the steps of (1) directing a thin stream of polishing electrolyte against one surface thereof while causing an electric current to flow through the electrolyte between the specimen as an anode and a cathode to remove a portion of the thickness of the specimen, and (2) terminating the flow of current and the flow of electrolyte in response to passage of electrolyte through a perforation in the specimen.

5. A process for thinning a foil specimen comprising the steps of (l) directing a thin stream of polishing electrolyte against one surface thereof while causing an electric current to flow through the electrolyte between the specimen as an anode and a cathode to remove a portion of the thickness of the specimen, (2) cleaning the polished surface of said one surface, (3) directing a thin stream of polishing electrolyte against the non-polished surface of the specimen while passing an electric current through the electrolyte between the specimen as an anode and a cathode, and (4) terminating the flow of current and the flow of electrolyte in response to passage of electrolyte through a perforation in the specimen.

6. The process of claim 5, wherein step (4) is accomplished by monitoring, with a snap action relay power amplifier, the electric current passing from the specimen to a sensing device positioned adjacent to but slightly apart from the said one surface.

7. A process for thinning a foil specimen comprising the steps of (l) directing a thin stream of polishing conductive fluid against one surface thereof while causing an electric current to flow through the conductive fluid between the specimen as an anode and a cathode to remove a portion of the thickness of the specimen, and (2) terminating the flow of current and the flow of conductive fluid in response to passage of conductive fluid through a perforation in the specimen.

53 3 UNITED STATES PATENT OFFICE CETIFICATE OFF CORRECTIQN;

3, 657, 083 Dated April 18, 1972 Jay Michael Larson, Douglas Hugh Polonis and Inventor(s) Raymond Taggart It is certified that error appears in the. above-identified patent and that said Letters Patent are hereby corrected as shown below:

Patent No.

Col. 1, line 1, insert the following paragraph:

The invention described herein was made under a grant from the National Science y of the United States Foundation, an agenc Government.--.

Signed and sealed this 3rd day of October 1972.

(SEAL) Attest: r

. v ROBERT GOTTSCHALK EDWARD MOFLETCHERJR. Attesting Officer Commissioner of Patents 

2. The apparatus of claim 1, wherein the means responsive to passage of electrolyte through the specimen for terminating both flow of current and the flow of electrolyte includes means for determining a voltage drop caused by the electrolyte establishing contact between the specimen and sensing means.
 3. The apparatus of claim 1 wherein the means responsive to passage of electrolyte through the specimen for terminating both flow of current and the flow of electrolyte comprises a conductive sensing device adapted to be positioned immediately adjacent one surface of the specimen and a snap action relay power amplifier connected with the sensing device.
 4. A process for thinning a foil specimen comprising the steps of (1) directing a thin stream of polishing electrolyte against one surface thereof while causing an electric current to flow through the electrolyte between the specimen as an anode and a cathode to remove a portion of the thickness of the specimen, and (2) terminating the flow of current and the flow of electrolyte in response to passage of electrolyte through a perforation in the specimen.
 5. A process for thinning a foil specimen comprising the steps of (1) directing a thin stream of polishing electrolyte against one surface thereof while causing an electric current to flow through the electrolyte between the specimen as an anode and a cathode to remove a portion of the thickness of the specimen, (2) cleaning the polished surface of said one surface, (3) directing a thin stream of polishing electrolyte against the non-polished surface of the specimen while passing an electric current through the electrolyte between the specimen as an anode and a cathode, and (4) terminating the flow of current and the flow of electrolyte in response to passage of electrolyte through a perforation in the specimen.
 6. The process of claim 5, wherein step (4) is accomplished by monitoring, with a snap action relay power amplifier, the electric current passing from the specimen to a sensing device positioned adjacent to but slightly apart from the said one surface.
 7. A process for thinning a foil specimen comprising the steps of (1) directing a thin stream of polishing conductive fluid against one surface thereof while causing an electric current to flow through the conductive fluid between the specimen as an anode and a cathode to remove a portion of the thickness of the specimen, and (2) terminating the flow of current and the flow of conductive fluid in response to passage of conductive fluid through a perforation in the specimen. 